/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineUserInterface.h
Get the cached user id for the specified external id
user info or null ptr if not found
FUniqueNetIdPtr GetExternalIdMapping ( const [FExternalIdQueryOptions](API\Plugins\OnlineSubsystem\Interfaces\FExternalIdQueryOptions) & QueryOptions, const [FString](API\Runtime\Core\Containers\FString) & ExternalId )